Internet Group Management Protocol (IGMP) SNOOPING refers to a procedure in which a communication device maintains local multicast forwarding list items by snooping IGMP Join messages or IGMP Leave messages from users so as to determine which member ports a multicast stream is to be forwarded to. During this procedure, the messages of the users are not modified and the communication device should forward uplink and downlink IGMP messages as they were.                In an application scenario of using IGMP SNOOPING in IGMP V3 application context (IGMP V3 SNOOPING), an end user may add sequentially programs of a plurality of Internet Service Providers (ISPs). The ISPs are identified by a Virtual Local Area Network (VLAN) or IP, and a terminal of the user may send a request message containing only one program for requesting the program, and may also send at a time a request message containing the respective (S, G)s of a plurality of programs (in the IGMP V3, each of the multicast programs is identified with an (S, G) in which S indicates an IP address of a source of the program and G indicates a group IP address of the multicast programs).        
After the end user orders the program(s) and a multicast router acting as a query device sends a general query message of the IGMP V3, the user terminal can carry a plurality of programs or only one program in a response message upon receipt of the general query message. In the case that a plurality of programs is carried in the response message, as a standard implementation of IGMP SNOOPING, the message will be forwarded to the plurality of ISPs as it was although it carries the plurality of programs. Accordingly, a server of each of the ISPs may receive the response message containing the plurality of programs. The programs in the message belong to different ISPs, and therefore the different ISPs receive the same message, resulting in information leakage. Meanwhile, there may be a hidden security risk. For instance, a program stream may be forwarded in error if the different ISPs identify a program with the same (S, G).